Axial compressive behavior of circular hollow steel tube-reinforced UHTCC column

To address material-related defects in concrete-filled hollow steel tube structures, this paper proposed a novel structural form: circular hollow steel tube-reinforced UHTCC columns. Axial compression tests were conducted on ten such columns, using hollow ratio and size effect as experimental parameters. This study investigated crack development, failure patterns, load-displacement behavior, and stress-strain relationships of the composite columns. The influence of various parameters on bearing capacity, strength, ductility, and energy dissipation was analyzed comprehensively. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to examine the failure modes at the interface between the UHTCC and the steel tube. Based on the experimental findings, an axial compression constitutive model for circular hollow steel tube-reinforced UHTCC columns was developed, providing formulas for each phase of the model. A finite element model was established using ABAQUS to analyze the stress distribution in each component. Furthermore, a formula for calculating the axial compressive capacity of these columns was proposed. The results indicated that under axial compression, all components functioned in unison, exhibiting coordinated deformation. As the size effect increased, the bearing capacity, strength coefficient, and energy dissipation significantly improved, whereas the ductility coefficient decreased. Conversely, as the hollow ratio increased, the bearing capacity, strength, and energy dissipation gradually declined, whereas the ductility coefficient increased. Microstructural analysis revealed that UHTCC exhibited a dense structure with strong adhesion to the steel tube. PVA fibers effectively controlled crack propagation in UHTCC, thereby enhancing both the load-bearing capacity and ductility of the composite columns. The proposed axial compression constitutive model for circular hollow steel tube-reinforced UHTCC columns demonstrated a high level of accuracy. Additionally, the calculated results for the axial compressive capacity closely matched both experimental and finite element data.